Vehicles having a series-parallel hybrid electric vehicle powertrain have two power sources for delivering driving power to vehicle traction wheels. In general, these power sources may be divided into two broad categories: mechanical and electrical. A mechanical power source may include, for example, an internal combustion engine that outputs mechanical power to the vehicle wheels through one or more sets of transmission gears. An electrical power source may include one or more electric machines, such as an electric motor or a motor/generator. The electric motor may receive electrical power from another generator, a battery, or other source of electrical power. The electric motor receives the electrical power from the electrical power source, and outputs mechanical power to the vehicle wheels generally through one or more sets of transmission gears.
Because this type of arrangement provides multiple power flow paths to the vehicle wheels, it may be possible to optimize the use of the various powertrain components to maximize system efficiency for a given driver demand for power. One such power management strategy is described in U.S. Pat. No. 7,398,147 issued on 8 Jul. 2008, which is hereby incorporated herein by reference.